The present invention relates to a coaxial cable, and more particularly to an improved low-loss coaxial cable having enhanced attenuation and mechanical bending properties.
Coaxial cables are commonly used today in the transmission of broadband signals, such as cable television signals and cellular telephone broadcast signals, for example. One typical type of coaxial cable includes a core containing an inner conductor, an aluminum sheath surrounding the core and serving as an outer conductor, and a foam polymer dielectric which surrounds the inner conductor and electrically insulates it from the surrounding metallic sheath. A protective jacket is often provided surrounding the metallic sheath.
Coaxial cable manufacturers continue to strive to improve the electrical performance of the cable, and in particular, to lower the signal attenuation at high frequency. At the same time, any alterations in the cable design must maintain adequate mechanical characteristics, such as cable bending performance and resistance to unwanted deformation during installation, which can impair the electrical performance. U.S. Pat. No. 4,104,481 addressed these concerns by improving the composition of the foam dielectric. U.S. Pat. No. 4,472,595 provided improvements in cable performance by reducing the stiffness of the tubular sheath in relation to the stiffness of the cable core.
The present invention provides an improved cable with excellent mechanical performance and with lowered attenuation at high frequency. In accordance with the present invention, the cable uses an outer tubular sheath formed of a bimetallic material of two different metals.
The cable comprises at least one inner conductor, a foam dielectric surrounding this inner conductor, and an electrically and mechanically continuous tubular sheath formed of a bimetallic material closely surrounding the foam dielectric and being adhesively bonded thereto. The bimetallic tubular sheath includes an inwardly facing layer of a first metal bonded to the dielectric and an outwardly facing layer of a second metal different from the first metal. The inwardly facing first metal layer preferably has a lower resistivity than the outwardly facing second metal layer.
The wall thickness of the tubular metallic sheath is suitably less than about 750 micrometers and the first metal layer may have a thickness less than about 100 micrometers. In a further more specific aspect, the first metal is copper and the second metal is aluminum.
The coaxial cable may further include a protective outer jacket surrounding the sheath. Preferably, the tubular metallic sheath has a thickness of no greater than about 2.5 percent of its outer diameter.
In one specific embodiment, the coaxial communications cable comprises a center conductor extending coaxially of the longitudinal axis of the cable and formed of a copper-clad aluminum bimetallic conductor, a low loss foam dielectric surrounding the inner conductor, and an electrically and mechanically continuous smooth-walled tubular sheath formed of a bimetallic material closely surrounding said foam dielectric. The bimetallic tubular sheath includes an inwardly facing copper layer and an outwardly facing aluminum layer metallurgically bonded to the copper layer. The sheath has a wall thickness of less than 750 micrometers and the wall thickness is no greater than about 2.5 percent of its outer diameter. A thin continuous layer of adhesive is disposed between the foam dielectric and the sheath and serves to bond the foam dielectric to the inwardly facing copper layer to form a structural composite. A polymeric jacket surrounds the tubular sheath and is bonded to the outwardly facing aluminum layer.
These and other features of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description which describes both the preferred and alternative embodiments of the invention.